Green tea contains volatile oils, vitamins, minerals, and caffeine, but the primary constituents of interest are
the polyphenols, particularly the catechin called epigallocatechin gallate (EGCG). The
polyphenols are believed to be responsible for most of green tea’s roles in promoting
Green tea has been shown to mildly lower total
cholesterol levels and improve the cholesterol profile (decreasing LDL “bad”
cholesterol and increasing HDL “good” cholesterol) in most,2
3 4 5 but not all,6 studies. Green tea may also promote
cardiovascular health by making platelets in
the blood less sticky.
Green tea has also been shown to protect against damage to LDL (“bad”)
cholesterol caused by oxygen.7 Consumption of green tea increases antioxidant
activity in the blood.8 Oxidative damage to LDL can promote atherosclerosis. While population studies have
suggested that consumption of green tea is associated with protection against
atherosclerosis,9 the evidence is still preliminary.
Several animal and test tube studies have demonstrated an anticancer effect of polyphenols
from green tea.10 11 12 In one of these studies, a polyphenol
called catechin from green tea effectively inhibited metastasis (uncontrolled spread) of
melanoma (skin cancer) cells.13 The polyphenols in green tea have also been
associated with reduced risk of several types of
cancer in humans.14 15 16 However, some human studies
have found no association between green tea consumption and decreased cancer
In a double-blind trial, people with
leukoplakia (a pre-cancerous oral condition) took 3 grams orally per day of a mixture of
whole green tea, green tea polyphenols, and green tea pigments orally, and also painted a
mixture of the tea on their lesions three times daily for six months.19 As compared
to the placebo group, those in the green tea group had significant decreases in the
Compounds in green tea, as well as black tea, may reduce the risk of dental
caries.20 Human volunteers rinsing with an alcohol extract of oolong tea leaves
before bed each night for four days had significantly less plaque formation, but similar
amounts of plaque-causing bacteria, compared to those with no treatment.21
Green tea polyphenols have been shown to stimulate the production of several immune system cells, and have topical antibacterial
properties—even against the bacteria that cause dental plaque.22
One study found that intake of 10 cups or more of green tea per day improved blood test
results, indicating protection against liver damage.25 Further studies are needed
to determine if taking green tea helps those with liver diseases.
Tea flavonoids given by capsule reduced
fecal odor and favorably altered the gut bacteria in elderly Japanese with feeding tubes
living in nursing homes.26 The study was repeated in bedridden elderly not on
feeding tubes, and green tea was again shown to improve their gut bacteria.27 These
studies raise the possibility of using green tea in other settings where gut bacteria are
disturbed, such as after taking antibiotics.
Further studies are needed to clarify the role of green tea in this respect, however.
High-tannin tea has been shown to reduce the need for blood removal from people with iron
overload, or hemochromatosis, in an open study.28 The tea had to be taken with
meals and without lemon or milk to be effective. Tea is believed to help in hemochromatosis by
preventing iron absorption.
In a double-blind trial, men with precancerous changes in the prostate received a green tea
extract providing 600 mg of catechins per day or a placebo for one year. After one year,
prostate cancer had developed in 3.3% of the men receiving the green tea extract and in 30% of
those given the placebo, a statistically significant difference.29 These results
suggest that drinking green tea or taking green tea catechins may help prevent prostate cancer
in men at high risk of developing the disease.
There are four case reports in which certain types of leukemia or lymphoma (low grade
B-cell malignancies) improved after the patients began taking green tea
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